Porous membranes have long been used to filter fine solids from fluids. Microfiltration, ultrafiltration, nanofiltration, and reverse osmosis (“RO”) are examples of processes based on the use of porous membranes. Applications in which these processes are employed include purifying salt water to produce drinking water, filtering wastewater for reuse as industrial process water, and removing unwanted solids from certain beverages such as beer and wine.
Microfiltration processes are generally used in applications in which it is desired to remove relatively large molecules from a fluid stream. Microfiltration generally operates at lower pressures than ultrafiltration and RO. Applications suited for microfiltration include, but are not limited to, waste water treatment, oil-water separation, and dust collection.
Ultrafiltration is a pressure-driven membrane process capable of separating solution components on the basis of molecular size and shape. Under an applied pressure difference across an ultrafiltration membrane, solvent and small solute species pass through the membrane while larger solute species are retained by the membrane. Typical applications for ultrafiltration include pretreatment of sea water in desalinization plants and treatment of wastewater for reuse as process water.
Reverse osmosis has found widespread use in filtration applications which require filtration of very fine solids, including dissolved ions. For example, in regions of the world with limited sources of fresh water, RO has been successfully used to purify sea water. Typically, each RO membrane in the apparatus is positioned within a tubular, outer, pressure vessel adapted to withstand the higher pressures associated with the RO process. The porous membrane used in the RO process is often bonded to or coated on a porous drainage layer to form a sandwich-like structure. Three of the four sides of this “RO membrane sandwich” are sealed. The fourth side of the sandwich is fed into a slot in a core and spirally wrapped around such core to achieve the desired surface area. The membrane of the RO sandwich is generally made from a material different from the material of the drainage layer.
In most cases, the porous membrane used in the aforementioned filtration processes is attached to a porous substrate. Such porous substrates may be very thin (e.g., from 15 μm to 95 μm in thickness) and, therefore, fragile and unable to provide structural support. See, e.g., Examples 1-3 and 9 of U.S. Pat. No. 4,828,772 to Lopatin et al. If the porous substrate is able to provide a mechanical support for the membrane used in the aforementioned filtration processes (thereby making the membrane more suitable for applications requiring, e.g., higher pressure), the substrate is made of a different material than the material from which the membrane is made. In general, the membrane and substrate are layered, or the membrane is anchored to the substrate. There is no bond between the membrane and substrate and there is a distinguishable interface between the membrane and substrate.
Some membranes used in ultrafiltration process are composite membranes. Composite membranes have reportedly been made using glycerin sandwiched between an ultrafiltration membrane and a microfiltration membrane substrate, each of which can be made from the same polymer. The glycerin acts to reduce the effect of the ultrafiltration membrane solution on the microfiltration membrane. See e.g., U.S. Pat. No. 4,824,568 to Allegrezza, Jr. et al. and published European Patent Application Number 0596411 A2 of Millipore Corp. These references disclose introducing glycerin and/or using a nonsolvent for the microfiltration membrane (which is on the order of 125 micrometers (μm) thick) when applying an ultrafiltration membrane solution to prevent etching and/or dissolving of the microfiltration membrane substrate by the ultrafiltration membrane solution and avoid fusing of the formed ultrafiltration membrane to the microfiltration membrane substrate, even if the membrane and the substrate are made from the same polymer.
The casting of a membrane made of one material on a substrate made of another material can yield materials poorly suited for many applications, especially when the membrane material and substrate materials have different solubility in the casting solvent and different thermal properties. For instance, the membrane surfaces of such dissimilar materials are sometimes not uniform. This lack of uniformity diminishes the strength with which the membrane is adhered to the substrate, and can lead to wide pore size distribution, leading to the uneven flow of liquids through the membrane, and unpredictable performance properties.
Another design consideration is that the different materials used as membrane and substrate exhibit different chemical and thermal properties. Therefore, the two different materials (e.g., two different polymers) generally have poor adhesion and significant voids at the interface between the membrane and substrate. This may be due, in part, to poor miscibility of the two different materials. Poor adhesion can also be due to differing thermal properties of the materials, which can lead to tension at their interface, causing delamination and surface cracking. The vulnerability of the existing membrane-substrate systems to delamination is exacerbated by the pressures used in filtration processes (high pressure is favored for increasing the flux during separations). Delamination in existing two-material, membrane-substrate systems is also caused by the frequent application of pressure used to backflush or backwash the system. Indeed, backflushing is one of the major causes of delamination in two-material, membrane-substrate systems used in microfiltration and ultrafiltration. Thus, it is desirable to provide materials that have strong adhesion between the membrane and substrate which can be used in a variety of filtration applications.
Additionally, there remain unmet needs for a porous membrane which can be steam-sterilized (existing membranes containing polyethylene cannot be steam-sterilized) and has greater backpressure resistance (which provides for better cleaning and extended membrane life).
Existing manufacturing methods for making membrane-coated tubular substrates can involve completely filling the bore of a vertically-oriented tube with a solution of the membrane material, allowing a weighted device inserted into the top of the interior to slowly sink to the bottom of the vertical tube and, as it does so, extruding the membrane solution through the porous surfaces or walls of the tube, and cleaning the outside of the tube with a ring-shaped device slightly larger then the outer diameter of the tube by sliding the ring-shaped device from top to bottom of the tube. Such methods are not optimal because, for example, they are time consuming, costly, and may result in a non-uniform membrane coating and an uneven depth of membrane penetration into the substrate tube.